10W LED outdoor lighting repair

[AlexK]

Hello,

My apologies in advance for the long post.

I've recently bought two 10W led outdoor lamps: One with white light and another with RGB.
http://www.ebay.com/itm/10W-LED-Waterproof-Outdoor-Spot-Light-Garden-Yard-Wall-Landscape-Path-Lighting-/191368225911?pt=LH_DefaultDomain_0&var=&hash=item2c8e6f6477#shpCntId

The description states that the operating voltage is 12-24VDC. So I connected the lamps to a 24V power supply. The white one is working fine. The RGB one, however, lit up for a few seconds, flashed a couple of time and died.

I took it apart to see if there are any visual signs of damage, but besides an input 25V rated (!) capacitor everything seemed fine. The construction is pretty simple: a pcb with the led driving circuitry and two wires going off to another pcb with 5 series LEDs on it.

As a first step I disconnected the LEDs from the driving circuit and measured the forward voltage. Turned out one of the LEDs was blown. I shorted it out and connected the 4 series LEDs directly to a power supply. Here are the numbers:

Voltage [V]	Current	Wattage [W]
10.5	0.24	2.5
11.5	0.44	5w
12.5	0.65	8.11
13	0.71	9.2

This is where I'm starting to get confused:
How can it be that the LEDs are already consuming 8W (recall that now there only 4 LED in series, 2W each) when the supply voltage is 12.5, but the entire thing is rated for 24V ?

Next, I turned to the LED driving circuit. It wasn't to hard to redraw the schematic (see attached).
The circuit uses the XL6001 constant current driver:
[url=http://www.kin-track.com/userfiles/down/XL6001%20datasheet.pdf]http://www.kin-track.com/userfiles/down/XL6001%20datasheet.pdf [/url]

This is where I'm getting even more confused: The feedback resistor, R1, is 1.1ohm, and according to the datasheet the feedback voltage is 0.22. So, the current should be about 220mA. But according to the table above, at 240mA the voltage is 10.5V.
How is it possible, given the fact that the XL6001 is a boost converter and the device is rate for 24V operation?

By the way, I tried to measure the actual feedback and output voltage but it was acting weird and changing with the power supply voltage. Seems like there is something wrong with the chip. Perhaps I'll try to use a different circuit based on a more common chip but first I'll appreciate any help in understating the operation principle of this LED driver.

Thanks for any help,
Alex

[EMUD]

Hi Alex,
you are right, this unit (if your schematic is ok) should not be used with 24 V. In my opinion it will work with voltages lower 5 times Uf_LED.
For your LEDs Uf is about 3V. The current is limited by the boost converter to 0.2A as vou mentioned, so the unit will have a power of 3W
to the LEDs.

If you raise the voltage to a higher llevel the LEDs will be destroyed by overcurrent and (or) overheat, because the current limitting factor is
your power supply or xformer ! In this case the boost regulator is off.

I have seen some chinese LED lamps labeled with 10W and a consumption of 4W ... its just nearly the same!

EMUD